1. Field of the Invention
The present invention relates to a method and to a device for measuring Joule-Thomson coefficients of fluids.
2. Description of the Prior Art
The Joule-Thomson coefficient .mu. measures the temperature variation of a fluid subjected to a pressure drop in an isenthalpic situation. ##EQU1##
This isenthalpy condition is precisely the one that is encountered during expansion of a fluid in a valve or in a line provided that the energy which is dissipated from the fluid can be disregarded.
Precise measurement of the Joule-Thomson coefficient finds applications in many fields where carrying fluids in pipes leads to changes of state that affect the proper circulation thereof, notably in the field of high-pressure/high-temperature (HP-HT) hydrocarbon reservoir production. This measurement allows determination of the &lt;&lt; thermal profile&gt;&gt;in all the energy dissipative elements.
According to the temperature and pressure conditions, the Joule-Thomson coefficient .mu. can be positive or negative, as shown in FIG. 1. In the case of a positive coefficient (.mu.&gt;0), the gas cools down during an expansion whereas a negative coefficient (.mu.&lt;0) leads to warming through expansion. The positive range of the coefficient is separated from the negative range by inversion curve IC. It can be seen that, under the HP-HT conditions prevailing in a well at a great depth, coefficient .mu. is negative: the fluid warms up through expansion. This is observed at the present time in reservoir production wells situated at a relatively great depth, notably in certain wells in the North Sea producing condensate gases where the HP-HT conditions cause an inversion of the Joule-Thomson coefficient.
The sign and the value of the Joule-Thomson coefficient are therefore important for dimensioning of a production well since it influences the thermal profile of the production facilities. In the case of a negative coefficient, it is imperative to know the warming reached through expansion: selection of the building materials depends thereon. This coefficient can also be used for dimensioning gas lines, for the same reasons. It is also necessary to know the sign and the value of this coefficient in order to assess the risks of hydrate or paraffin formation in case of a temperature decrease through expansion, so as to be able to select the suitable technique allowing prevention of the formation of deposits in the lines.
There are many reference books in the literature showing how to calculate the inversion curve IC(P,T) of the Joule-Thomson coefficient by means of equations of state conventionally used in the petroleum industry. One can notably refer to:
Kortekaas W. G., et al; Joule-Thomson Expansion of High-Pressure-High-Temperature Gas Condensates, in Fluid Phase Equilibria, 139, 1997, p.207-218.
However, this approach is difficult to exploit in practice for lack of the necessary experimental data which are scarcely disclosed. Furthermore, measuring the Joule-Thomson coefficient .mu. is delicate because very low absolute values of the order of some tenths .degree. C./bar (some .degree. C./MPa) are assessed. Good determination of the inversion curve IC requires great precision because the observable temperature difference is very close to 0.
There are different types of experimental devices allowing determination of the Joule-Thomson coefficient by measuring the temperature variation of a fluid flowing through an element.
According to a first embodiment, this element consists of a porous medium causing a pressure drop that is a function of its permeability and of the fluid flow rate. Such a device has many drawbacks insofar as the Joule-Thomson effect is dispersed in the whole porous volume that is difficult to insulate thermally, and it has a certain thermal inertia, which requires a large amount of fluid in order to reach the state of thermal equilibrium during measurement.
According to a second embodiment, the element causing a pressure drop consists of a valve. This is an advantageous solution because, in this case, the pressure difference between the inlet and the outlet of the device can be readily varied. Furthermore, the Joule-Thomson effect is rather localized, but the various parts of the device such as the seat, the needle, etc., however form a thermal mass producing thermal losses that are difficult to prevent.
Assurance of a good thermal insulation and of a good localization of the Joule-Thomson effect are the key factors of a good measurement. These are the qualities of the device according to the invention.